210 BONY FISHES vn. 12 



receptors sensitive to light. The minnow Phoxinus has a transparent 

 patch on the head in this region, and it has been found possible to 

 train the fish to give appropriate responses to changes of illumination 

 even after removal of the paired eyes and the pineal body. Evidently 

 there are light-sensitive cells in other parts of the walls of the dien- 

 cephalon, besides those that become evaginated to form the eyes. 

 Experiments on lampreys also showed the presence of such cells 

 (p. 105). The hypothalamus is well developed and receives large tracts 

 from the forebrain. Below and behind it is a large saccus vasculosus 

 in some forms (p. 169). 



The midbrain is often the largest part of the brain. The cells spread 

 out over its roof (tectum opticum) are not all collected round the 

 ventricle but have migrated away to make an elaborately layered 

 system. Into this midbrain cortex there pass not only the great optic 

 tracts but also ascending tracts from the sensory regions of the spinal 

 cord, lateral line system, gustatory systems, and cerebellum. Large 

 motor tracts pass back towards the spinal cord; the details of their 

 endings have not been traced, but they certainly exercise control over 

 motor functions. Electrical stimulation of the optic lobes produces 

 well-coordinated movements of local groups of muscles, for instance 

 those of the eyes or fins. It can hardly be doubted that this well- 

 developed midbrain apparatus thus controls much of the behaviour of 

 the fish and is able to mediate quite elaborate acts of learning and 

 other forms of more complex behaviour. After removal of the tectum 

 of one side a minnow is blind in the opposite eye. Each part of the 

 retina is mapped on to a distinct area of the tectum and if the optic 

 tract is cut and allowed to regenerate this projection is exactly replaced. 

 When a goldfish is trained to respond to some visual stimulus the 

 learning process occurs in the midbrain and continues unaffected after 

 removal of the forebrain. Conversely, olfactory learning takes place 

 in the latter and is undisturbed by injury to the tectum opticum. 



The base of the midbrain (tegmentum) contains motor centres. 

 Electrical stimulation here produces abrupt and massive responses of 

 the locomotor apparatus, very different from the sequences of co- 

 ordinated movements that appear after stimulation of the roof of the 

 tectum. 



The cerebellum is very large in teleosts, especially in the more 

 active swimmers, and a forwardly directed lobe of it, the valvula 

 cerebelli, extends under the midbrain. Various disorders of movement 

 have been reported after removal of the cerebellum, such as swaying 

 when moving quickly. Presumably it plays an important part, as in 



